This invention relates to an improved filter holding device for sealably mounting a filter in a supporting structure. The improved holding device is particularly adapted for use with filters of the type that are made in the form of a filter pack. The invention works in conjunction with such filters to supply virtually contaminant-free air to critical production or laboratory areas such as clean rooms or white rooms.
In the present age of advanced technology, there is an increasing requirement for particulate-free and/or bacteria-free environments. These environments are typically referred to as clean rooms and to provide such clean rooms with the required contaminant-free air, it is a practice to circulate the air through a bank of one or more filters. In many clean rooms, the efficiency required in removing particles from the air must exceed 99.99% of particles larger than 0.3 microns. This high level of particulate filtration is achieved by the use of what are in the industry generally called "absolute" or "HEPA" filters. These filters are comprised of suitable filtering media, having corresponding efficiencies, which are placed in a filter frame and sealed with adhesives to form filter packs. The filter pack is in turn typically mounted and sealed into a somewhat rigid supporting structure.
A known method of sealably mounting the filter packs into the support structure comprises a gasket of neoprene or rubber or other similar material that is placed between the filter pack and the supporting structure or holding device. The gasket runs around the periphery of the filter pack and is designed to contact a "knife-edge" peripheral flange of the holding device. A clamping mechanism provides a force to press the filter pack and its gasket against the knife-edge to prevent the leakage of air that is unfiltered, and hence contaminated, from entering the clean room.
This gasket sealing method that is used by many filtering applications becomes unsatisfactory because the neoprene gasket or other rubbers tend to deteriorate or lose their elasticity with the passage of time. This presents a serious problem since the seal around the perimeter of the filter pack may now allow unfiltered air past the filter and into the clean room environment, thereby defeating the original purpose of filtering.
Another sealing method comprises a filter pack being surrounded and supported in the usual manner by a holding device. The downstream face of the holding device comprises a continuous circumferential or peripheral channel that is initially filled with a fluid having a high consistency at room temperature such as household petrolatum. The sealed frame of the filter pack has a downstream protruding peripheral edge that corresponds to the channel in such a manner that when the filter pack is mounted in the holding device the protruding edge of the filter pack frame enters the fluid-filled channel and results in a more or less absolute air seal between the two members.
However, although the fluid-seal approach appropriately performs its function of preventing unfiltered air leaks, the initial cost of the petrolatum and the inconvenience in installing and maintaining the fluid in the channels makes this method highly unsatisfactory.
The present invention provides a filter holding device that combines with a filter pack to entirely eliminate unfiltered air leakage around the filter pack. Further, this invention eliminates unfiltered air leakage around the filter pack inexpensively and efficiently. More particularly, this invention provides a filter holding device that does not rely on household petrolatum or other similar fluid as a sealing element.
The present invention has the advantages of providing a filtering assembly that functions with a conventional filter pack to entirely eliminate unfiltered air leakage around the filter pack into a clean room and yet which allows the filter pack to be easily and quickly assembled in, or removed from, the filter holding device. Moreover, it is an advantage of the present invention to provide a filtering system that is quick and efficient to initially install in a clean room environment.
Accordingly, this invention relates to an improved filter holding device for both sealing and supporting a filter pack. The holding device combines with the periphery of the filter pack to form a chamber between the filter pack and the holding device. This chamber is subjected to a low or negative pressure relative to the pressure outside the chamber, thus creating a vacuum within the chamber so that if any unfiltered air leaks past a sealing means on the upstream side of the filter pack and housing device, is is prevented from entering and contaminating the clean room.
The vacuum is attained by connecting the chamber surrounding the filter pack frame to the air return or an additional blower located on the upstream side of the filter pack. In the event a seal is not working properly, thereby allowing a contaminant to pass into the chamber, the vacuum action will pick up the contaminant and carry it back to the upstream side of the filter where it can be discarded or re-filtered so as to prevent no risk of contaminating the clean room.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying description.